Among the major problems influencing the outcome of human marrow transplantation are disseminated cytomegalovirus(CMV) infection, leukemic relapse, and graft-versus-host-disease(GVHD). Sixty of the 377 marrow graft recipients developed CMV pneumonia with an 88% mortality. Incidence was 22% among those with leukemia in relapse at the time of transplant. Graft-versus-host-disease occurs in up to 70% of allogeneic graft recipients, and relapse of leukemia occurs in 50% of patients with leukemia. Overall, the one year survival is 30%. Human leukocyte interferon has properties that could be beneficial for all 3 conditions. It is antiviral, has been shown to ameliorate GVHD in animal models, and has had promising preliminary results against neoplastic disease in both animals and man. This proposal outlines a prospective randomized trial of human leukocyte interferon after allogeneic marrow transplant for leukemia in relapse, in an attempt to prevent or ameliorate these important complications of marrow transplantation.